Additional studies have extended our knowledge of the physical and biological properties of rabbit reticulocyte initiation factors. Specifically, eIF-2 has been shown to be a trimer which contains three dissimilar subunits. The high molecular weight initiation factor, eIF-3, has been estimated to have a molecular weight of 850,000. The most recent studies have attempted to define the role of initiation factors in the recognition of mRNA. Of the three factors required for natural mRNA translation, eIF-4B appears to be most responsible for the differential recognition of capped mRNAs. Studies on the control of protein synthesis have indicated that phosphorylation of eIF-2 by the hemin controlled repressor (HCR) does not alter eIF-2 function. Preliminary data has been obtained on the isolation of elongation factors which correspond to the prokaryotic factors EF-Tu and EF-Ts. Three such factors have been found: an approximately 50,000 dalton form which binds Phe-tRNA to ribosomes; a form containing multiple subunits of approximately 30,000 daltons; and a form containing, at least, a 30,000 dalton and 50,000 dalton peptide. The latter two forms stimulate the first form in polyphenylalanine synthesis, but not, as yet, in the binding of Phe-tRNA to the ribosome.